1. Field of the Invention
The present invention relates generally to cellular networks, and specifically to carriers within cellular networks.
2. Background and Objects of the Present Invention
Code Division Multiple Access (CDMA) utilizes a spread spectrum technique, in which channels and communications are transmitted (spread) over a single frequency band with several mobile stations (MSs) simultaneously using the frequency band. For example, a single CDMA carrier within an IS-95 system usually has a frequency band associated with it that is 1.23 MHz wide, and typically supports approximately twenty subscribers simultaneously. As another example, within a Wideband CDMA (WCDMA) system, a single carrier has a 3.84 MHz frequency band associated with it that typically serves a lot more than twenty subscribers simultaneously.
Each signal transmitted on either the uplink (from the MS to the base station) or the downlink (from the base station to the MS) consists of a different pseudorandom binary sequence (hereinafter referred to as a code) that modulates the carrier and spreads the spectrum of the waveform. Thus, each carrier contains a number of different codes that can be allocated to form different user bit rates. Some of the codes are used for Traffic channels, while others are used for control channels.
Both traffic channels and control channels can be mapped on either dedicated channels or common channels, some of which are power controlled and some of which are non-power controlled. The dedicated channels transfer dedicated information to a particular MS. The downlink transmitted power is optimized for each channel, according to the distance between the base station and the MS and the interference experienced by the MS. An example of such a channel is the Dedicated Channel (DCH) in the WCDMA system. Non-power controlled common channels have a fixed downlink power allocated in order to support coverage over a specific area, such as beyond the borders of the cell. Examples of such channels are the Pilot channel of IS-95 systems and the Synchronization Channel (SCH), Broadcast Channel (BCH) and Paging Channel (PCH) or WCDMA systems. Power controlled common channels are shared by many MSs, but the transferred information is dedicated for each MS. Therefore, with power controlled common channels, there can be a separate downlink power control for each MS-connection. An example of such a channel is the Forward Access Channel (FACH) in WCDMA systems.
In every CDMA system, there are a certain number of downlink code channels allocated as control channels. For example, in the IS-95 system, there are 64 potentially available downlink code channels, nine of which are reserved for control information. To increase the number of available Traffic Channels, the IS-95B version of CDMA was introduced. In the IS-95B version, an additional uplink and downlink carrier is added with a reduced set of control channels. A first downlink carrier consists of all of the control channels, and a second carrier consists of only the Pilot channel of the control channels and up to 63 downlink Traffic channels.
However, the additional uplink carrier is symmetrical to the additional downlink carrier (equal air interface bandwidth on both). Thus, even with the addition of a second downlink carrier, the MS is still restricted in that it can only allocate Traffic channel(s) from one of the carriers during a call. In addition, since the Pilot channel has a high downlink transmit power associated with it to support general coverage over a specific area (typically beyond the border of the cell), by keeping the Pilot channel on the second carrier, the transmit power allocated to Pilot channel will force the downlink transmit power allocation of Traffic channels to significantly higher power levels, due to downlink interference. This increases the interference in the cell and increases the size and weight of Multi/Single Carrier Amplifiers used in CDMA cells.
Furthermore, within any type of cellular system, including CDMA and Time Division Multiple Access (TDMA) systems, the carriers for the uplink and downlink directions each contain an equal amount air-interface bandwidth. Thus, the same amount of spectrum is typically used for both directions for a call. Although a user may be able to establish a non-symmetrical connection for certain services defined by the operator, the cellular systems of today do not have the flexibility required to allow an operator to allocate non-symmetrical bandwidth over the air-interface for any call.
For example, many service providers are now offering wireless Internet connections to MSs. However, Internet browsing requires significantly more downlink bandwidth than a normal call. Unfortunately, it is currently not an efficient usage of resources to connect a non-symmetrical bandwidth call (allocate more downlink bandwidth than uplink bandwidth) out of a symmetrical spectrum. If an operator were to do so, a portion of the uplink bandwidth corresponding to additional allocated downlink bandwidth would not be used.
It is, therefore, an object of the invention to provide for multiple carriers within cellular network cells.
It is a further object of the present invention to allow for flexible allocation of bandwidth on multiple carriers within a cell.
It is still a further object of the present invention to utilize a different number of carriers for downlink and uplink bandwidth.
It is still a further object of the present invention to allow downlink bandwidth to be allocated from more than one carrier for a particular mobile station.
It is still a further object of the present invention to provide for a primary carrier containing all types of channels and a secondary carrier containing only dedicated channels and optionally power-controlled common channels.
The present invention is directed to telecommunications systems and methods for efficient and flexible usage of bandwidth within a code division multiple access (CDMA) cell. Multiple carriers can be included within a CDMA cell, in which a different number of carriers can be allocated to downlink transmissions and uplink transmissions. In addition, a mobile station (MS) can allocate downlink bandwidth from different carriers to contribute to a Traffic channel, making it possible to form high user bit-rates, e.g., 4 Mbps. To reduce the total transmit output power within a cell, on the downlink, the cell can contain a Primary carrier with at least non-power controlled common channels and at least one Secondary carrier with only dedicated channels and optionally power-controlled common channels. The Primary carrier has a primary scrambling code and zero or more secondary scrambling codes for scrambling transmissions over a first CDMA bandwidth, while the Secondary carrier consists of one or more secondary scrambling codes for scrambling transmissions over a second CDMA bandwidth. The primary scrambling code is used to scramble non-power controlled channels as well as power-controlled channels, while the secondary scrambling code is used to scramble only power-controlled channels (common or dedicated).